1. Field of the Invention
The invention relates to plasma generation systems that are controlled using Gravity-Induced Gas-Diffusion Separation (GIGDS) techniques.
2. Description of Related Art
Typically, during semiconductor processing, a plasma etch process is utilized to remove or etch material along fine lines or within vias or contacts patterned on a semiconductor substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a process chamber. In addition, during semiconductor processing, a plasma enhanced chemical vapor deposition (PECVD) process can be utilized to deposit material to fill trenches, vias, and/or contacts patterned on the semiconductor substrate.
For example, in plasma etch processes, once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure. Thereafter, a plasma is formed when a portion of the gas species present is ionized following collisions with energetic electrons. Moreover, the heated electrons serve to dissociate some species of the mixture of gas species and create reactant specie(s) suitable for the exposed-surface etch chemistry. Once the plasma is formed, any exposed surfaces of the substrate are etched by the plasma. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the exposed regions of substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), poly-silicon, and silicon nitride, for example.
Conventionally, various techniques have been implemented for exciting a gas into plasma for the treatment of a substrate during semiconductor device fabrication, as described above. In particular, (“parallel plate”) capacitively coupled plasma (CCP) processing systems, or inductively coupled plasma (ICP) processing systems have been utilized commonly for plasma excitation. Among other types of plasma sources, there are microwave plasma sources (including those utilizing electron-cyclotron resonance (ECR)), surface wave plasma (SWP) sources, and helicon plasma sources.
SWP sources are known to offer improved plasma processing performance, particularly for etching processes, over CCP systems, ICP systems and resonantly heated systems. SWP sources produce a high degree of ionization at a relatively lower Boltzmann electron temperature (Te). In addition, SWP sources generally produce plasma richer in electronically excited molecular species with reduced molecular dissociation. However, the practical implementation of SWP sources still suffers from several deficiencies including, for example, plasma stability and uniformity.
In current semiconductor process, electronegative gases (e.g. O2, NO, N2O, Cl2, HBr, F2, SxFy, CxFy, CxFyHz, or their mixtures, etc.) are often added into electropositive gases such as N2 or inert gases (e.g. Ar) for etch, deposition, and cleaning. Due to the interaction between electropositive ionization (produce positive ions and electrons) and electronegative ionization (produce positive ions and negative ions along with small amount electrons), the balance between electron attachment and electron detachment may not continue. Together with other known or unknown reasons, plasma generation using mixtures of electronegative gas and electropositive gas have many kinds of problems and issues related to process control and quality.
Instability can be a problem in a plasma source. For example, plasma generation can be unstable and this instability may manifest as plasma “flickering”, or the plasma source may not be tuned at certain process conditions when using a mixture of electronegative gas and electropositive gas. The instability problem can influence the process performance by restricting the process window, affecting plasma uniformity, reducing productivity by adding stabilization time, or may even cause device failure, etc.
Electromagnetic (EM) radiation can be a problem in a plasma source. For example, the electron density in the plasma is lower when one or more electronegative gases are added. As a result, the EM wave would not be blocked (absorbed) by plasma (plasma electrons), and the EM wave would propagate to the wafer area, or areas with sensitive devices attached to the plasma chamber. Such EM radiation could damage the wafer thereby causing device and process failure, or adversely affect the sensitive devices attached to the plasma chamber. Therefore, the plasma process has to be restricted to electron over-dense conditions, such as higher power or specific ranges of pressure, and this limits the process window and adds energy cost.
Uniformity can be another exemplary problem in a plasma source. In some cases, the electron density and ion density may not be uniform because the electronegative discharge is strongly dependant on the electrical field intensity. For example, there can be strong electronegative discharge with very low electron density, or there can be totally electropositive discharge with very low negative ion density, or there can be other types of discharges that can be between the two extreme conditions, depending on pressure, partial pressure (or flow rate ratio) of electronegative gas to electropositive gas, and power. If, at the plasma generation region, the electric field is not uniformly distributed, then a non-uniformly distributed electronegative discharge region and electropositive discharge region can cause a non-uniform electron density and ion density. This non-uniformity may also affect plasma stability.
Erosion and contamination problems can also exist in a plasma source when the plasma is generated near the plasma-dielectric interface. The reactive and corrosive electronegative gas or other process gas cause dielectric plate erosion by chemical reaction and/or by physical sputtering and contamination. This induces not only plasma generation and control problems, but also reduces the lifetime of the dielectric plate, and requires extra plate cleaning processes and/or replacement. In addition, this erosion and contamination would increase particle density that may cause device/wafer failure.
The above are only several examples that would be induced by adding electronegative gas or other process gases in the plasma process chamber. The problems are not limited to those examples and are not limited to microwave plasma source, meaning that those problems can be associated to any plasma source and processes.